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Demersal

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Guest post: Lauren Flynn, New Mexico State University

Example of a demersal catch, painted in the 17th century. Nature morte de poissons au bord d’un ravage by Jan van Kessel the Elder, Source: Wikimedia Commons.

From the Latin verb demergere, meaning ‘to sink’, demersal describes fish that live on or just above the ocean or lake floor.  It can also refer to a commercial fishery for groundfish and shellfish, which typically uses gear that is weighted so it sinks to the bottom.  Demersal fish are frequently captured by a trawl net that drags along the ocean or lake floor.  As a result, demersal fisheries tend to be less discriminate than pelagic fishing methods such as long lines or gill nets that target fish using specific bait, specialized hooks, or mesh size.  Bycatch can be an issue for demersal fisheries, but trawl net modifications using Bycatch Reduction Devices (BRD) help to increase capture of target species and reduce unintended catch.

Examples of demersal fish species are cod, haddock, and flatfish such as turbot and halibut.  Popular shellfish include crab, shrimp, and lobster.

For comparison, see pelagic.

Diadromous

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George S. Myers defined the term diadromous to refer to fish that migrate between fresh and salt water.

George S. Myers defined the term diadromous to refer to fish that migrate between fresh and salt water.

Diadromous fish migrate between fresh and salt water.  This term, coined by George S. Myers in 1949, refers to both anadromous and catadromous fishes.  Diadromous life history strategies have evolved in numerous fish taxa.  The complex life cycles do require complex osmoregulatory adaptation but the advantages of the given environments for specific life stages of the fish outweigh the costs.

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Dorsal fin

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The dorsal fin is a stabilizer fin

The dorsal fin is a stabilizer fin

The dorsal fin is located on the top of a fish (as well as some marine mammals).  Predominately, the fin is used to stabilize fish in the water and help direct the fish through turns and stops.  Some fish, like Striped Bass (Morone saxatilis), have two dorsal fins and others have very highly derived dorsal fins, such as the Ocean Sunfish (Mola mola) which uses its dorsal fin as a primary mode of locomotion.

The Ocean Sunfish has a highly modified dorsal fin

The Ocean Sunfish has a highly modified dorsal fin

The dorsal fin of sharks is one of the fins targeted in shark finning operations and destined for shark fin soup and traditional Chinese medicines.  Shark fin soup is a delicacy in the Chinese culture, served at special occasions, including weddings.  As China has become more affluent, the demand for luxury items, such as shark fin soup, has dramatically risen.

Dorsal fins of sharks are targeted for shark fin soup

Dorsal fins of sharks are targeted for shark fin soup